The present invention relates to a mass spectrometer that includes an improved quadrupole mass analyser arrangement. The invention will be described mainly with reference to an inductively coupled plasma-mass spectrometer (ICP-MS) having an inductively coupled plasma ion source, however it is to be understood that the invention encompasses other types of mass spectrometers employing other types of ion sources, examples of which are disclosed hereinbelow.
The subject disclosure refers to a mass spectrometer having an ion reflecting or an ion transmissive optics system. The spectrometer includes an ion source for providing a supply of particles including ions representative of chemical elements present in an analytical sample and an ion optics system between the ion source and a mass analyzer for producing a beam of ions from the source and establishing a reflecting electrostatic field for reflecting ions form the beam through an angle, for example 90xc2x0, and for focussing them into the mass analyzer entrance.
It has been found that the present invention as embodied in an ICP-MS instrument gives excellent sensitivity for detection of elemental isotopes having relatively high atomic masses (for example, the sensitivity for thorium, atomic mass 232, was over 650,000 counts per second per microgram per liter). However the sensitivity for elemental isotopes having low atomic masses is relatively poor (for example the sensitivity for beryllium, atomic mass 9, was less than 10,000 counts per second per microgram per liter). Furthermore, the background count rate (the count rate detected at a selected mass-to-charge ratio when no ions having that selected mass-to-charge ratio where expected to be present) was higher than desired, and when the voltages applied to the ion optics electrodes were increased to improve the focussing to increase sensitivity for detection of low atomic mass isotopes, the background count rate unfavourably increased.
The best possible Limit of Detection (LOD) for an elemental isotope in an ICP-MS is given by
LOD=3xc3x97(background count rate/measurement time)1/2/sensitivity 
Thus the relatively high background count rates and relatively low sensitivities for elemental isotopes having low atomic masses means that detection limits for such low atomic mass isotopes are undesirably high.
Although this problem has been highlighted by use of a mass spectrometer which employs a reflecting ion optics system, it is considered (in view of what is thought to be the mechanism for causing the high background count rates, as explained hereinbelow) that the same problem would exist in mass spectrometers that do not use a reflecting ion optics system.
It is known to arrange a separate set of four short straight sections of rod at the entrance of a quadrupole mass analyser and operate them with only radio-frequency (rf) voltage applied thereto or with the ratio of the DC to AC voltage substantially zero. Such a set of rods is often known as xe2x80x9cfringe rodsxe2x80x9d because their function is to alleviate the effect of the fringing fields at the entrance of a quadrupole mass analyser and so improve the efficiency of transmission of ions into the mass analyser (see Peter H Dawon""s book xe2x80x9cQuadrupole Mass Spectrometry and its Applicationsxe2x80x9d, Elsevier Scientific Publishing Co., 1976, at p. 105 and FIG. 1(b); and the earlier disclosure of U.S. Pat. No. 3,371,204 (Wilson M Brubaker)). While these straight fringe rods are not directly related to the problem of excessive background in quadrupole mass spectrometry, similar structures have been involved in efforts to solve that problem.
Thus U.S. Pat. No. 3,473,020 (Wilson M Brubaker) discloses a quadrupole mass filter having a curvilinear entrance section and a rectilinear section. A charged particle source directs particles (normally ions) into the analyser where they are resolved and the sorted beam is then directed into a detector section. The curvilinear quadrupole section can be operated in a strong focussing mode with low resolving power such that ions in a small mass range are transmitted from this section into the quadrupole rectilinear section of high resolving power. The curvilinear entrance section also reduces the number of photons from the charged particle source reaching the analyser detector and thus provides a substantial improvement in the signal to noise ratio in the output of the analyser. This arrangement would also remove neutral particles emanating from the source as well as photons because these particles would not be affected by the electrostatic field in the curved quadrupole section and so would continue straight ahead and strike the curved electrode rods. In a subsequent U.S. Pat. No. 3,410,997, Brubaker discloses the use of a similar curved quadrupole section at the exit of a linear quadrupole mass analyser to separate ions from photons from the source. It is disclosed that this curved quadrupole section may be operated with AC voltages only.
Peter H Dawson in his above mentioned book xe2x80x9cQuadrupole Mass Spectrometry and its Applicationsxe2x80x9d at pp 34-35 describes that background signal limits the ability to measure trace concentrations and originates from excited neutrals which easily pass through the xe2x80x9cline-of-sightxe2x80x9d analyser. He goes on to describe that xe2x80x9ccurved quadrupoles . . . or curved sections . . . have also been used to avoid the problemxe2x80x9d.
European Patent Application 0 237 259 A2 (J. E. P. Syka) discloses tandem quadrupole mass spectrometer arrangements that include a bent quadrupole placed in front of a mass analysing quadrupole for reducing output noise. This bent quadrupole removes fast neutral particles generated in the ion source or from a collision cell (for producing daughter ions) in front of the bent quadrupole. In Syka""s invention the bent quadrupole is separated from the mass analysing quadrupole by aperture plates and electrostatic lenses. The bent quadrupole does not act as a set of xe2x80x98fringe rodsxe2x80x99.
D. J. Douglas in his article xe2x80x9cSome Current Perspectives on ICP-MSxe2x80x9d (Canadian Journal of Spectroscopy, Vol. 34, No. 2, 1989, pp 38-49) reported, in relation to seeking to reduce the high level of background noise in inductively coupled plasma mass spectrometry, the use of a curved (90xc2x0) RF only quadrupole (which he terms a xe2x80x9cbent quadxe2x80x9d) at the exit of the analysing quadrupole, which is essentially the same arrangement as that disclosed by Brubaker in U.S. Pat. No. 3,410,997. Douglas states, however, that the background noise (i.e. count rate) was a strong function of mass, that is, for high mass ions the background was reduced dramatically, but for low masses the background remained high (which is similar to the problem described hereinbefore in relation to the invention of WO 00/17909). Douglas describes, xe2x80x9cApparently at the exit of the analysing quadrupole, photons or metastable atoms from the source were somehow producing low mass ions which were efficiently transmitted to the detector to produce a high background level. When the voltage on the RF quad was high (corresponding to high mass analytes) these low mass ions had unstable trajectories and were not transmitted. Thus the xe2x80x9cbent quadxe2x80x9d almost but did not quite solve the background problemxe2x80x9d (ibid p.41).
U.S. Pat. No. 5,939,718 (N. Yamada et al) discloses an ICP-MS having an ion lens section, including a multipole (at least four electrode rods) ion beam guide located in front of mass filtering and ion detection sections. In some embodiments (FIGS. 9-12) the rods of the ion beam guide are tilted or bent with respect to the moving direction of an ion beam xe2x80x9cso as to prevent an (sic) direct entrance of photons of light from an inductively coupled plasma into (the) mass filter . . . Consequently the noise from direct light can be reduced . . . and it can highly enhance the S/N ratio and the measurement accuracy.xe2x80x9d Thus this patent addresses a problem that is essentially the same as that addressed in U.S. Pat. No. 3,473,020 (Brubaker) and claims a solution that is generally similar, but specifically applied to an inductively coupled plasma mass spectrometer.
According to the disclosure in Yamada et al. U.S. Pat. No. 5,939,718, the bent ion guide is separated from the mass analysing quadrupole by an aperture plate. The bent ion guide therefore does not act as a set of xe2x80x98fringe rodsxe2x80x99. Because of this aperture the mass filter in Yamada et al. U.S. Pat. No. 5,939,718 does not directly receive ions from the ion guide. Instead the ion guide is located in an ion lens vacuum chamber and the mass filter in an analyser vacuum chamber such that the ions must pass through an aperture between the two chambers. Such an aperture plate would introduce distortions in the electric fields associated with the ion guide and the mass filter which, together with different vacuum levels in the two chambers, may cause some unwanted effects on the ions and thus contribute to the background noise (particularly in view of what is thought to be the mechanism for causing the high background count rates in relation to the invention of WO 00/17909, as explained hereinbelow).
The above disclosed prior art documents show the use of curved or tilted ion guides to remove unwanted particles (i.e. neutrals and photons) which emanate from a source. The effect of such ion guides is to locate the mass filter and/or ion detector xe2x80x9coff-axisxe2x80x9d or out of a xe2x80x9cline-of-sightxe2x80x9d from the ion source. They do not address the problem of a high background count rate still occurring in an arrangement in which neutrals and photons emanating from a source have already been removed.
An object of the present invention is to provide a mass spectrometer that employs a quadrupole mass analyser which has an improved (that is, a low) limit of detection for elemental isotopes of low atomic masses. The mass spectrometer may employ either a transmissive or reflecting ion optics system.
According to the invention there is provided a mass spectrometer including,
a source for producing particles including ions representative of chemical elements in a sample together with neutral particles and photons,
an ion optics system contained in a first vacuum region for receiving particles from the source, the ion optics system including
at least one first electrode for establishing an electrostatic field for directing a beam of said ions in a first direction from the source and at least one second electrode for establishing an electrostatic field for diverting the beam of ions from the first direction through an angle whereby neutral particles and photons emanating from the source continue in the first direction and are separated from the beam of ions,
a quadrupole mass analyser arrangement contained in a second vacuum region and including
a set of quadrupole fringe electrodes for receiving the beam of ions and a linear quadrupole mass analyser for receiving ions directly from the set of quadrupole fringe electrodes, and an ion detector also contained in the second vacuum region for receiving ions from the linear quadrupole mass analyser,
wherein the set of quadrupole fringe electrodes are configured to divert the ions prior to their passage into the linear quadrupole mass analyser and to shield the entrance of the linear quadrupole mass analyser.
It has been discovered that the use of a configured set of quadrupole fringe electrodes immediately in front of a linear mass analyser as disclosed in the preceding paragraph and after neutrals and photons from the source have been removed, significantly improves the limit of detection for elemental isotopes of low atomic masses. This is principally because the configured set of quadrupole fringe electrodes of the quadrupole mass analyser arrangement have the effect of reducing the background count rate to a very low figure, even when the voltages of the preceding ion optics elements are set to values that favour the transmission of isotopes of low atomic masses. Without the set of quadrupole fringe electrodes the background count rate at such voltages is unacceptably high. Use of the configured set of fringe electrodes thus permits an increase in sensitivity for low mass isotopes along with a decrease in the background count rate. Both these factors contribute to the improved limits of detection for isotopes of low atomic mass.
It is thought that the reduction of the background count rate is due to the configured quadrupole fringe electrodes preventing the entry of energetic neutral particles into the linear quadrupole mass analyser, such energetic neutral particles possibly being produced by acceleration of the sample ions through residual gas in the spectrometer, which can occur whether those sample ions are directed by either a transmissive or reflecting ion optics system. Whatever the origin of the species causing the high background may be, it is clear that in the case of the invention disclosed in International Application WO 00/17909 these species cannot come directly from the ion source, as has been taught in the prior art. Accordingly, it is thought that acceleration of the ions in the second direction through the residual gas in the first or second vacuum regions causes some of those ions to interact (for example by resonant charge exchange) with atoms of the residual gas and so produce high energy neutral atoms which, were they to enter the linear quadrupole mass analyser, would interact with metal surfaces that they might strike and so generate ions that pass into the ion detector, thus increasing the background count rate. The configuration of the quadrupole fringe electrodes section of the mass analyser arrangement therefore is such that it causes a diversion of the sample ions that is sufficient to prevent entry of so produced high energy neutral atoms into the linear qaudrupole mass analyser section. That is, the configuration of the set of quadrupole fringe electrodes is such that any ions that may happen to be neutralised will continue in a ballistic trajectory that results in them striking a fringe electrode and so prevent them from reaching the ion detector.
Thus the electrodes of the set of quadrupole fringe electrodes are configured to divert the sample ions from their travel in an entry direction of the ions into the set of quadrupole fringe electrodes prior to their passage into the linear quadrupole mass analyser, and which shield the mass analyser entrance as viewed in the entry direction so as to prevent neutral particles, possibly created by passage of the ion beam in the entry direction through residual gas in the first or second vacuum regions, from entering the linear quadrupole mass analyser.
Furthermore, the ions upon passage through the set of quadrupole fringe electrodes of this invention pass directly into the linear quadrupole mass analyser. That is, the configured set of quadrupole fringe electrodes and the quadrupole electrodes of the linear mass analyser are contained in the same vacuum region and are thus both kept at the same low pressure to minimise collisions of ions with the background gas. Thus this feature of the invention establishes conditions between the configured set of quadrupole fringe electrodes and the linear mass analyser, namely the absence of a pressure gradient and a uniform electrostatic field distribution, which reduce the opportunity for production of the high energy neutral particles which it is thought contribute to the problem that is addressed by the present invention. This structure is contrary to that disclosed by the Yamada et al Patent U.S. Pat. No. 5,939,718.
It is considered there could be two components to the motion of any energetic neutral particle that might have been formed by resonant charge exchange between a high-velocity ion and the background gas. The more obvious component would lie along the direction of travel of the ion beam as it entered the space defined by the set of quadrupole fringe electrodes. The other, less obvious, component would lie along the direction of travel that the ion was following at the instant that the charge exchange occurred. Ions travelling through a space defined by the set of quadrupole fringe electrodes are subject to sinusoidal acceleration by a radiofrequency electromagnetic field applied to the fringe electrodes. This sinusoidal acceleration has a component in a direction perpendicular to the path lying along the geometric centre of the set of fringe electrodes, as defined by the point of intersection of the two lines connecting the centre of one electrode of each pair to that of the diametrically opposite electrode. The orientation and configuration of the set of quadrupole fringe electrodes with respect to the trajectory of the incoming ion beam is chosen to shield the ion detector from neutral particles having either of the two possible components of motion just described.
Preferably the beam of ions directed in the first direction is diverted from this direction through an angle and in a second direction. The magnitude of this angle is such that there is effectively no possibility of light or any other particles (other than ions) from the source reaching the detector. It is considered that an angle of more than 10xc2x0 is required for this. Preferably the angle is substantial, for example, an angle of about 90xc2x0 may be employed. Alternatively the ions may be diverted through an angle to bypass a neutral stop and then refocussed into a beam after passing the neutral stop such that they continue substantially in the first direction.
Preferably a first set of electrodes is provided for establishing the electrostatic field for directing the beam of ions in the first direction and preferably a second set of electrodes is provided for establishing the electrostatic field for diverting the beam of ions from the first direction and in a second direction. Preferably the second at least one electrode or set of electrodes is for establishing a reflecting electrostatic field for reflecting the beam of ions from the first direction into the second direction thereby separating said reflected ions from neutral particles and photons from the source which continue through the reflecting electrostatic field and are removed. Use of such a reflecting electrostatic field allows for very efficient removal of such neutral particles and photons.
Preferably the set of quadrupole fringe electrodes comprise four elongate electrodes which are curved to thereby define a curved diversionary path for the ions Alternatively non-curved electrodes may be provided, for example electrode rods which are tilted as described herein below may be provided.
Preferably, with curved elongate quadrupole fringe electrodes, the electrodes are configured such that the ions exit the set generally in the same direction along which they enter the set of electrodes. Thus it is advantageous to configure the set of curved quadrupole fringe electrodes in such a way that the entrance end and the exit end thereof are substantially parallel but not co-linear, being joined by a gently curved section that is approximately the shape of a distorted letter xe2x80x98sxe2x80x99. Other configurations are possible so long as the ions are focussed through an aperture and enter the set of quadrupole fringe electrodes in front of the linear mass analyser, the fringe electrodes being so configured that they act to guide the ions along a path that is different from that followed by neutral particles entering the mass analyser arrangement. Such neutral particles are thereby prevented from entering the linear quadrupole mass analyser and subsequently producing ions that would be detected and contribute to the background count rate.
Preferably the electrodes of the set of quadrupole fringe electrodes are configured such that, viewed in the direction of entry of ions into the fringe electrodes, the electrodes at least cover the linear mass analyser and thus the ion detector entrances. That is, the orientation of the curved quadrupole fringe electrodes is such that if at any place the direction of curvature of an electrode is such that an ion accelerated by the RF fields applied by the electrodes might be accelerated in the direction of the ion detector, an electrode portion lies between the accelerated ion and the entrance of the linear mass analyser and thus the detector. This ensures that the ion detector lies in the shadow of a fringe electrode in the event that an accelerated ion becomes a neutral particle by resonant charge exchange with the background gas. This provides very efficient shielding of the ion detector from neutral particles.
For a better understanding of the invention and to show how it may be carried into effect, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.